This invention relates generally to pressurized fluid-containing devices and, more particularly, to such a device used in the inflation of an inflatable device such as an inflatable vehicle occupant restraint airbag cushion used in inflatable restraint systems.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an "airbag cushion," that is inflated/expanded with gas when the vehicle encounters sudden deceleration, such as in a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as "an inflator."
Many types of inflator devices have been disclosed in the art for inflating an inflatable restraint system airbag cushion. One category of such inflator devices is often referred to as "compressed gas inflators" and refers to various inflators which contain compressed gas.
In one particular type of compressed gas inflator, commonly referred to as a "stored gas inflator," the inflator simply contains a quantity of a stored compressed gas which is selectively released to inflate an associated airbag cushion. Unfortunately, to properly inflate a typically sized airbag cushion at an appropriate rate, such an inflator commonly requires the storage of a relatively large volume of gas at relatively high pressures. As a result of such high storage pressures, the walls of the gas storage chamber of such an inflator are typically relatively thick for increased strength. The combination large volume and thick walls typically results in a relatively heavy and bulky inflator design. In addition, the operation of such an inflator device requires the design and provision of a technique to initiate the release of the stored gas into the airbag cushion, when desired.
In a second type of compressed gas inflator, commonly referred to as a hybrid inflator, inflation gas results from a combination of stored compressed gas and the combustion of a gas generating material, e.g., a pyrotechnic. Hybrid inflators that have been proposed heretofore have been subject to certain disadvantages. For example, such inflators commonly result in the production of a gas having a relatively high particulate content. The removal of such solid particulate material, such as by the incorporation of various filtering devices within or about the inflator, undesirably increases the complexity of the inflator design and processing and can increase the costs associated therewith.
In view of these and other related or similar problems and shortcomings of prior inflator devices, a new type an inflator, called a "fluid fueled inflator," has been developed. Such inflators are the subject of commonly assigned U.S. Pat. No. 5,470,104, Smith et al., issued Nov. 28, 1995; U.S. Pat. No. 5,494,312, Rink, issued Feb. 27, 1996; and U.S. Pat. No. 5,531,473, Rink et al., issued Jul. 2, 1996, the disclosures of which are fully incorporated herein by reference.
Such an inflator device utilizes a fuel material in the form of a fluid, e.g., in the form of a gas, liquid, finely divided solid, or one or more combinations thereof, in the formation of an inflation gas for an airbag cushion. One form of the fluid fuel inflator utilizes a compressed gas. In such a fluid fueled inflator, the fluid fuel material is burned to produce gas which contacts a quantity of stored pressurized gas to produce inflation gas for use in inflating a respective inflatable device.
While such an inflator can successfully overcome, at least in part, some of the problems associated with the above-identified prior types of inflator devices, there has been a continuing need and demand for further improvements in safety, simplicity, effectiveness, economy and reliability in the apparatus and techniques used for inflating an inflatable device such as an airbag cushion.
To that end, the above-identified pending application, U.S. Ser. No. 08/632,698, filed on Apr. 15, 1996, discloses a new type of inflator wherein a gas source material undergoes decomposition to form decomposition products including at least one gaseous decomposition product used to inflate an inflatable device.
Such an inflator can be helpful in one or more of the following aspects: reduction or minimization of concerns regarding the handling of content materials; production of relatively low temperature, non-harmful inflation gases; reduction or minimization of size and space requirements and avoidance or minimization of the risks or dangers of the gas producing or forming materials undergoing degradation (thermal or otherwise) over time as the inflator awaits activation.
In general, all inflators (including pyrotechnic-based inflators) have specific requirements and thus necessitate the checking of the inflators, or at least particular components thereof, for the presence of undesired leaks.
For example, in pyrotechnic inflators, the gas generating material may often be susceptible to undesirably absorbing water, such as from the ambient environment. Since pyrotechnic inflators are generally not pressurized, there is a potential for atmospheric moisture to diffuse into such an inflator. Thus, assuming the inflator is in a humid surrounding environment and the existence of a leak path into the inflator, moisture could potentially be absorbed into or by the generant. As a result, the inflator may not perform as optimally as desired, particularly if the inflator has been exposed to elevated amounts of moisture. Consequently, pyrotechnic inflators generally contain internal seals to prevent or minimize such entry of water or moisture into the device. During manufacture of the inflator, these internal seals are checked for the presence or occurrence of undesired leak paths.
Compressed gas inflators, such as described above, commonly require the presence of at least certain specified quantities of the compressed material in order for the inflator to perform in the designed for manner. In such inflators, it is generally desired that the amount(s) of stored compressed material(s) be maintained in the inflator within at least a certain tolerance in order to ensure proper operation of the inflator. While proper inflator operation can be variously defined, ultimately, an inflator and the associated airbag need provide adequate vehicle occupant protection over an extended period of time (typically 15 years or more) after original construction of the vehicle. Thus, beyond simple functioning of the inflator and deployment of the associated airbag, the airbag desirably deploys in the desired and proper manner.
There are various methods to determine the leakage rate of a compressed gas inflator. In practice, a typically preferred method involves the use of helium as a tracer gas in a compressed gas mixture. In such a method, a certain fraction of the composition of the stored gas which escapes from the inflator consists of helium. (The exact fraction of helium detected as a result of the leak may be equal, less than, or greater than the corresponding loading conditions of the originally stored compressed gas. The physics associated with these various situations, however, is beyond the scope of the present discussion. In general, however, these different situations are typically dependent on certain, particular factors such as the magnitude of the leak, the total pressure within the storage vessel, as well as the initial gas composition, for example.)
The leak rate of helium from a pressure vessel is normally detected using a mass spectrometer system. For this specific practice, the mass spectrometers are normally designed to detect the presence of helium in the gases constituting the sample. The utilization of helium in leak tracing is advantageous in several respects: a) First, since the presence of helium is rather rare in the atmosphere, background helium (or residual helium in the environment such as that surrounding the detection apparatus) is normally very low. As a result, the possibility of the mass spectrometer being falsely influenced and possibly producing a spurious signal is significantly reduced or minimized. b) Second, the mass spectrometer signals for certain different molecular species can be nearly the same. Consequently, the mass spectrometer signal produced or resulting from the presence or occurrence of one molecular species may interfere or mask the mass spectrometer signal produced or resulting from the presence or occurrence of a different molecular species. For example, the molecular weights of nitrous oxide and carbon dioxide are approximately 44.02 and 44.01, respectively. As a result, it is very difficult to distinguish between these molecular species via mass spectrometry. Helium, however, with a molecular weight of 4, produces a mass spectrometer signal that is relatively easily distinguishable from the signal produced by other potentially present species. c) Third, helium is a relatively small monatomic gas, facilitating the passage thereof through even relatively small or narrow leak paths.
Conventional helium leak detection techniques, however, suffer or potentially suffer from a number of problems or disadvantages. For example, in order to permit leak check to the relatively small range of leakage acceptable in airbag module inflators, it is commonly necessary to include relatively large amounts of helium in the compressed gas mixture. In practice, the amount of helium required will be dependent on factors such as the magnitude and type of leak, the design life of the inflator, and the criteria for adequate performance for the inflator as a function of time. However, the incorporation of even moderate amounts of helium within a compressed gas inflator is or can be disadvantageous as, for a given volume, the storage pressure of the contents is significantly increased. Conversely, at a given pressure, the storage volume needs to be increased in order to accommodate the mass of the added helium.
While the release of such stored helium would also normally contribute to the inflation of the associated airbag, the storage of a compressed gas mixture of two or more molecular species is typically more expensive than the storage of a single compressed gas molecular species. The use of two or more molecular species commonly necessitates the use of additional storage, handling, and mixing equipment.
A significant limitation on such use of helium in such leak detection schemes is that the leak rate from a pressure vessel normally cannot be accurately checked at a date substantially later than the date the inflator is manufactured, unless the helium concentration within the vessel is known. That is, unless the leak is of the type that the compressed gases (e.g., both the primary stored gas and the helium tracer gas) are escaping in equal proportion to that at which they were loaded (as in the original composition), then the leak rate determination will normally be in error. Since knowledge of the type of leak cannot be definitively known a priori, the making of such an assumption can result in significant error. Moreover, if a pressurized vessel is returned at a later date for the leak rate to be reevaluated, a helium leak rate determination may be inaccurate.
An additional possible limitation or drawback to the use of such helium leak detection techniques is that the occurrence or presence of liquid materials within the storage vessel may impede or "mask" the helium. For example, if a liquid with a relatively high surface tension is present in the vessel, such liquid could possibly flow into a hole through which gas would normally leak and may, at least temporarily inhibit the passage of the gaseous material leak passage and out of the inflator. However, with time, the liquid may no longer occupy the leak path and the stoppage of gas leakage therethrough may only be temporary.
In addition, though helium is relatively rare in the general atmosphere, it will be appreciated that relatively high background concentrations of helium can be created in manufacturing environments. This may necessitate that the tested vessel be isolated such as by being placed in a closed chamber in which a vacuum is created in the surrounding environment, with the helium leak rate then being determined. Such special handling can add significant time and expense to the manufacturing process.
Further, the use of helium may undesirably result in the addition of considerable expense to the cost of the inflator, both through the inherent cost of the helium itself, the cost of purchasing and maintaining the mass spectrometers, as well as the costs associated with the equipment required to store, mix, and handle the helium.
Thus, there is a need and a demand for a pressurized fluid-containing inflator design which facilitates leak detection.
Moreover, there remains a continuing need and a demand for an inflator device which satisfies one or more of the following objectives: increased simplicity of design, construction, assembly and manufacture; avoidance or minimization of the risks or problems associated with the storage, handling and dispensing of gas generant materials; permits even further reductions in assembly weight and volume or size; and realizes enhanced assembly and performance reliability.